1. Field of Invention
This invention relates to communications systems. Specifically, the present invention relates to oscillators and their associated periodic signals used in communications systems.
2. Description of the Related Art
Periodic electronic signals are used in a variety of demanding applications including reference signals used to modulate and demodulate signals in analog circuits and clocks for digital circuits. Such applications often require very accurate low-noise signals that can be generated with minimal power consumption and maintained accurate over a range of frequencies.
The accuracy of such signals is particularly important in digital communications systems such as code division multiple access (CDMA) systems or time division multiple access (TDMA) systems. CDMA communications systems are characterized by a plurality of mobile transceivers in communication with one or more base stations. Each transceiver includes a transmitter and a receiver. The transceivers require an accurate and precise reference frequency with low frequency drift characteristics to communicate with a base station. Crystal oscillators are often used to provide the requisite reference frequencies. Unfortunately, the frequency of the signals output by crystal oscillators tend to drift with time and temperature.
To account for time and temperature frequency drift, voltage controlled, temperature compensated crystal oscillators (VC-TCXOs) are often employed. A VC-TCXO is typically an open-loop device that includes a temperature sensing circuit that outputs a control signal to a frequency tuning circuit connected to a crystal oscillator. The temperature sensing circuit uses stored information corresponding to the crystal oscillator's frequency versus temperature profile to generate the control signal. The frequency tuning circuit uses the control signal to stabilize the frequency of the VC-TXO in response to changes in temperature.
Unfortunately, VC-TCXOs often include additional automatic frequency control (AFC) loops and analog tuning devices which occupy valuable circuit board space and consume power. Also, VC-TCXOs are typically expensive and have limited ability to compensate for oscillator frequency drift due to oscillator aging.
Another accurate but expensive oscillator is the digitally compensated crystal oscillator (DCXO). DCXOs require an expensive dedicated controller that must be calibrated for the crystal oscillator that it is designed to control. In addition, DCXOs typically have limited ability to compensate for age-related oscillator frequency drift.
To compensate for aging, an open-loop system employing a microprocessor and a voltage controlled oscillator (VCO) is often employed. Long-term oscillator frequency drift characterization data is stored in memory. The microprocessor accesses the characterization data and applies an age-compensating signal to the VCO. This system is often expensive, requiring additional firmware, memory, processors, and frequency drift characterization data.
Hence, a need exists in the art for a system having minimal space and power requirements for compensating for long-term frequency drift characteristics of an oscillator for use in communications systems.